Man arrested in US over ricin letters

A LETTER addressed to President Barack Obama has tested positive for ricin and authorities have arrested a suspect, stoking alarm in Washington after an earlier scare over poisoned mail.

The FBI said there was no connection between the blasts at the Boston Marathon that killed three people on Monday and mail sent to Obama, Republican Senator Roger Wicker and an unidentified Mississippi justice official.

Special agents on Wednesday afternoon arrested Paul Kevin Curtis, "the individual believed to be responsible for the mailings of the three letters sent through the US Postal Service," the bureau said.

The letters "contained a granular substance that preliminarily tested positive for ricin," it added in a statement.

Earlier reports had said the letters to Obama and Wicker, which never reached them, were signed "I am KC and I approve this message."

Curtis was arrested at his home in Corinth, Mississippi, according to the FBI.

After preliminary tests on the Obama letter showed traces of ricin, further tests would be carried out in the next 24 to 48 hours, the FBI said.

The US Secret Service said the letter to Obama had been intercepted at a mail screening site on Tuesday, the same day authorities said a letter sent to Wicker also showed traces of ricin.

Ricin - a highly toxic protein found in castor beans - can, when inhaled, cause respiratory problems. Ingested orally, it is lethal in even minuscule quantities.

Senator Carl Levin issued a statement saying one of his staffers had discovered a "suspicious-looking letter" at a regional office in Michigan and handed it over to authorities for further investigation.

The staffer had no symptoms, but was being held in hospital overnight as a precaution, the senator said.

Adding to nervousness in the US capital, two Senate office buildings were briefly cordoned off amid reports of a suspicious package but were reopened after an all-clear was given.

US Capitol Police confirmed one man was being questioned, but he was not being detained.

A mysterious series of letters laced with anthrax was sent to politicians and some journalists, killing five people and sickening 17 others, following the September 11 attacks in 2001.

Congressional mail has been screened off-site ever since.

Three Senate office buildings were shut in 2004 after tests found ricin in mail that had been sent to the Senate majority leader's office.

The biological agent was also sent to the White House and the Department of Transportation in November 2003. There were no injuries in those incidents.